We have recently observed that HIV-infected children, living in an urban setting in the northeastern United States, have decreased bone mass compared to healthy children. As childhood and adolescence are the critical periods during life for skeletal development and, as it is widely recognized that optimal bone mass accretion during childhood and adolescence may prevent osteoporosis later in life, this observation has serious implications for future risk of osteoporosis and fractures in HIV-infected children. While Highly Active Antiretroviral Treatment (HAART) has been implicated in the etiology of the skeletal demineralization in adults, it remains possible that other factors important for skeletal health, such as vitamin D deficiency, could contribute to or exacerbate bone loss in HIV-infected children. In this regard, we have found that 50% of a small group of HIV-infected children have deficient or insufficient body stores of vitamin D, as measured by serum 25-hydroxyvitamin D levels below 20 ng/ml. The central hypotheses of this proposal are that the prevalence of vitamin D deficiency and insufficiency is increased in HIV-infected children receiving HAART and is related to the decreased bone mass observed in these children. We also hypothesize that supplementation with vitamin D and the recommended daily allowance of calcium will improve bone mineral accrual in these patients, as it does in healthy individuals. We will test these hypotheses in 1. a crosssectional study comparing HIV-infected and healthy children and adolescents with respect to relationships between HIV-infection, vitamin D status, and bone mass, and in 2. a two-year randomized, controlled clinical trial in which the effects of vitamin D and calcium supplementation on bone mass accrual are compared to placebo in HIV-infected children and adolescents.